Articles made of aluminum or aluminum alloy, are customarily manufactured by a metal-forming operation called drawing and ironing. In the course of this and similar metal-forming operations a lubricant oil is applied to the surface of the metal being deformed, and some abraded aluminum particles and other contaminates (usually referred to as “smut”) adhere to the metal surface, especially to the inner walls of such beverage containers. The surfaces are protected by subsequent chemical-conversion coating and/or paint coating techniques. Therefore, the above-mentioned lubricant oil or smut must be removed, by cleaning, from the metal surfaces before the chemical-conversion coating.
This surface cleaning is normally applied by means of an acidic cleaning agent, which appropriately etches the metal surfaces. The currently used acidic cleaning agents used for smut-removal have generally been ones containing chromic acid or hydrofluoric acid. Especially, the cleaning agent containing the hydrofluoric acid is superior in enabling the low-temperature acidic cleaning (e.g., up to 50° C.).
U.S. Pat. Nos. 4,728,456 and 4,851,148 disclose a cleaning agent including an acidic cleaning agent of pH 2 or below prepared from sulfuric acid and nitric acid containing little or no fluoric ions with the addition of ferric ions serving an accelerator instead of fluoride ions, and a method for controlling the oxidation-reduction potential of the cleaning bath to control the ferric ion concentration in the bath, respectively.
PCT published application WO 93/01332-A1 discloses an acidic cleaning solution containing sulfuric acid and/or nitric acid and ferric ions serving as an accelerator for etching instead of fluoride ions, and further containing oxidized ion of diphenylamine having color-change potential (that is, at a transition of a certain potential, color becomes transparent) in the vicinity of standard oxidation-reduction potential (+0.77+/−0.09 V) where ferric ions (Fe3+) are changed into ferrous ions (Fe2+), oxidized ions of diphenylbenzidine and oxidized ions of sulfonic diphenylamine, and the cleaning process for controlling the ferric ion concentration by controlling the color-change point.
U.S. Pat. No. 3,607,484 discloses is a corrosion liquid consisting of sulfuric acid aqueous solution with the addition of metals (ions of Cu, Fe, Ni, Co, Sn, Zn, etc.) having a smaller ionization tendency than aluminum and 7 g ion/L of at least one selected from halogen ions (F−, Br−, I−) besides Cl−, PO43−, pyrophosphoric ion, pentaphosphoric ion and so on.
Japanese Patent Publication No. 47-39823 discloses a corrosion liquid containing 0.1 to 7.0 g ion/L of at least one of Cl−, F−, Br−, I−, phosphoric ion, pyrophosphoric ion, pentaphosphoric ion and so on.
Generally, the etching reaction of aluminum within the acidic cleaning solution includes an anode reaction in which aluminum is changed into aluminum ions (Al3+) and a cathode reaction in which H+ in the cleaning solution is reduced into ½ H2. Thus, the addition of ferric ions (Fe3+) into the acidic cleaning solution simultaneously causes a cathode action for reducing Fe3+ into Fe2+ and the reduction of H+, which accelerates the etching reaction of aluminum.
Further, the oxidizing agent is used to control the oxidation-reduction potential to control the ferric ion concentration within the bath, thereby suppressing the Fe2+ concentration which increases accordingly as the etching reaction advances and oxidizing the Fe2+ into Fe3+.
It is however known that the oxidizing agent typically acts to oxidize and decompose the surfactant. Therefore, the addition of an oxidizing agent into an acidic cleaning aqueous solution containing a surfactant for improving the degreasing ability may cause accumulation of oxidized decomposed substance within the cleaning bath, which will lead to a reduction in the degreasing ability on the aluminum surfaces. On the contrary, the addition of excessive oxidizing agent in order to maintain the degreasing ability will increase the operating cost.
In PCT published application WO 91/19830-A1 there is proposed an “acidic liquid composition and process for cleaning aluminum” containing a mineral acid selected from the group of phosphoric acid, sulfuric acid, and nitric acid, multiply charged metallic ions, surfactant, and oxidizing agent for oxidizing the multiply charged metallic ions which were reduced during the cleaning operation, with the addition of 0.05 to 5 g/l of a C2 to C10 glycol for suppressing the decomposing reaction of surfactant due to the oxidizing agent.
In the case of using the acidic cleaning agent disclosed in U.S. Pat. Nos. 4,728,456 and 4,851,148, however, the treatment must be made at a higher temperature (70° C. to 80° C.) than the temperature (up to 50° C.) of acidic cleaning by means of acidic cleaning agent containing fluoric ions in order to obtain the same effect as the acidic cleaning by the acidic cleaning agent containing fluoride ions, which will be economically disadvantageous. Since a multiplicity of Fe3+ ions are contained, a precipitation derived from ferric ions is produced, and in particular, iron hydroxide which is in the form of a precipitate may adhere to the heater section. Also, in the case of WO 93/01332-A1, it is necessary to perform acidic cleaning at high temperature, which will be economically disadvantageous.
The corrosion liquid disclosed in U.S. Pat. No. 3,607,484 and Japanese Patent Publication No. 47-39823 mainly aims to etch the aluminum alloy by electrodeposition in order to form a photoengraving. In the case of coexisting with the copper ion, as disclosed by U.S. Pat. No. 3,607,484, the oxidation-reduction potential is over 1.08 V in the etching treatment. Therefore, the use of Br ions as halogen ions besides Cl would lead to the reaction 2 Br−→Br2+2 e, which leads to the production of harmful bromine gas. Thus, exclusive treatment facility must be provided, which will be economically disadvantageous. In addition, these corrosion liquids contain 56 g/l or more of bromide ions for its object in the examples, which is different in the object of etching from the present invention.
In the acidic cleaning aqueous solution disclosed in WO 91/19830-A1, the content of a C2 to C10 glycol for the suppression of decomposition reaction of surfactant by the oxidizing agent is 0.05 to 5 g/l (namely, 50 to 5000 ppm) within the acidic cleaning aqueous solution, and hence the glycol compounds do not solely have the etching accelerating effect. Reversely, a large volume of addition will increase the effective ingredients, which will increase the load of liquid waste treatment.
The present invention was conceived in view of the above conventional problems, of which an object is to provide an acidic cleaning solution for aluminum and aluminum alloy and its cleaning process.